The present invention relates to a digital transmission system for information signals and particularly, but not exclusively, to a TDMA signal transmission system.
TDMA transmission systems are known per se and two examples are the Digital European Cordless Telephone Standard (DECT) and the digital cellular mobile telephone standard GSM (Groups Specials Mobile). In DECT, for example, each frequency channel is divided into successive concatenated frames and each frame comprises 24 time slots of which the first 12 time slots are allocated to transmission in say the forward direction from a base station and the remaining 12 time slots are allocated to transmission in the reverse direction. The nth (where n is an integer between 1 and 12) and the (n+12)th slot are allocated to one call. The pair of allocated time slots is termed a duplex voice channel and dynamic channel allocation is used by a cordless telephone and/or a base station to obtain a duplex voice channel.
In other TDMA transmission systems, a system controller controls channel allocation but this requires the transmission of system control messages which may be sent in a dedicated time slot in each frame or in a traffic time slot (or channel). For a digital trunked private mobile radio system it has been proposed that 25 kHz carrier spacing be used. However 25 kHz carrier spacing is incompatible with 12.5 kHz carrier spacing used for some analogue systems. Accordingly there is a desire to have a flexible multiple standard TDMA transmission system capable of operating on at least 25 kHz and 12.5 kHz carrier spacings and also of using different modulation schemes to provide different capacities.